CT of the chest and abdomen in patients on mechanical pulmonary ventilators: quality of images made at 0.6 vs 1.0 sec.

Objective: Chest and abdominal CT scans using 1.0-sec scan times are often limited by motion in patients who are unable to hold their breath. With our scanner we can obtain images in 0.6 sec (partial scan) that use data from 225 degrees instead of the 360 degrees used for 1.0-sec scans. The purpose of this study was to assess whether the quality of images of the chest and abdomen in patients on mechanical pulmonary ventilators who could not breath-hold could be improved if images were taken using a scan time of 0.6 sec rather than 1.0 sec.

Materials And Methods: Thirty patients who were being treated with mechanical pulmonary ventilation with indications for chest or abdominal CT or both were scanned with a scan time of 1.0 sec. At the end of the examination, additional 0.6-sec images were taken at three or four levels. Images obtained with these two scan times were filmed at similar levels and windows, and anatomically matched levels were compared. The images were masked and independently evaluated by three radiologists for motion, noise, artifact, and overall image quality. Each parameter was rated using a scale of 1 to 4. Using Kendall's tau correlation, there was no significant difference between the radiologists in the grading of individual parameters or of overall image quality. Therefore, the average of the scores of the three radiologists was used. Statistical analysis was done using repeated measures multivariate analysis of variance.

Results: Images obtained in 0.6 sec had significantly less motion (p < .001) but more noise (p < .001) than those obtained in 1.0 sec. We found no statistically significant difference in artifacts between the two scan times. Overall image quality was judged to be significantly better on scans obtained in 0.6 sec than on scans obtained at 1.0 sec (p < .001), in spite of the greater noise on 0.6-sec images. The higher quality of 0.6-sec images was most noticeable for lung window settings.

Conclusion: Our results indicate that scans taken in 0.6 sec (partial scans) provide better quality images than those obtained in 1.0 sec in patients being treated with mechanical pulmonary ventilation who cannot breath-hold. This technique may be useful not only in this population but in all patients who are unable to cooperate with breath-holding instructions.